1. Field Of The Invention
This invention relates to an improved method of separating glycols from crude glycol mixtures additionally containing dialkyl esters of such glycols. More particularly, this invention relates to a method of separating 1,4-butanediol from a crude 1,4-butanediol-containing mixture obtained as a result of hydrogenation of a mixture of dialkyl maleate and dialkyl fumarate.
2. Description Of The Prior Art
1,4-BUTANEDIOL IS A MONOMER USED IN THE PRODUCTION OF POLYBUTYLENE TEREPHALATE, A UNIQUE ENGINEERING PLASTIC. Butanediol has been manufactured commercially by processes involving the Reppe reaction of acetylene and formaldehyde followed by hydrogenation of the reaction mixture. Other processes for the production of 1,4-butanediol involve the hydrolysis of dichlorobutanes or the hydrogenation of maleic anhydride.
Processes for the production of 1,4-butanediol from maleic acid or maleic anhydride generally involve esterification of the maleic precursor to a dialkyl ester from which 1,4-butanediol is obtained by hydrogenation. The crude butanediol product obtained by hydrogenation of such dialkyl esters of maleic acid contains the desired 1,4-butanediol in admixture with unconverted dialkyl maleate, dialkyl fumarate and other by-products which are formed in the hydrogenation of the ester and which are impurities in the 1,4-butanediol product.
Recovery of substantially pure 1,4-butanediol by conventional distillation is impractical since the unreacted ester will co-distill with the desired diol product. Also impurities contained in the crude butanediol product obtained from the hydrogenation reaction such as alkyl succinate and alkyloxy alkyl succinate form azeotropic mixtures with the desired 1,4-butanediol. For example, dibutyl succinate and 1,4-butanediol form an azeotrope containing 40% dibutyl succinate. Likewise, dibutyl butoxy succinate forms an azeotrope with 1,4-butanediol containing approximately 10% dibutyl butoxy succinate. Therefore, 1,4-butanediol cannot be obtained in high purity from feedstocks containing such impurities using conventional distillation techniques.